Dictionary Wiki
When you say "I know that," what exactly are you claiming? Epistemology is the philosophical discipline that pokes at that question until it breaks open. It studies what knowledge is, where it comes from, how far it reaches, and what separates a well-founded conviction from a lucky guess or a stubborn opinion. The questions are old — Plato was already worrying about them — but they feel urgent again whenever algorithms, headlines, and expert disagreements force us to ask who to trust. The vocabulary below gives students, teachers, and curious readers the precise language philosophers use when they pick these problems apart.
Table of Contents
1. Core Terms to Start With
Every conversation about knowledge sits on a small stack of foundational words. Before arguments about skepticism or evidence make sense, the basic labels need to be clear.
Epistemology — The area of philosophy devoted to knowledge itself: its sources, its boundaries, its structure, and the conditions under which a belief earns the title of knowledge.
Knowledge — On the classical reading, a true belief that the believer has good reason to hold; more generally, a firm cognitive grip on how things actually are.
Belief — A person's mental endorsement of a proposition as true. Beliefs can be accurate or mistaken, well-supported or baseless; they are the raw material epistemology evaluates.
Justification — Whatever makes holding a belief rational: the reasons, evidence, or reliable processes behind it. It is the ingredient that turns a correct hunch into something worth calling knowledge.
Epistemic — An adjective meaning "having to do with knowledge or the conditions for having it." The root is the Greek episteme, understanding.
With these pieces in place, harder questions — about certainty, doubt, and the difference between knowing and merely feeling sure — become possible to ask precisely.
2. What Knowledge Is Made Of
Trying to define knowledge looks easy until you try. Philosophers have spent more than two thousand years refining the attempt, and each fix has invited a sharper counterexample.
Justified true belief (JTB) — The textbook formula: you know that p when p is true, you believe p, and you have adequate grounds for believing p. For centuries this counted as the working definition.
Gettier problem — Edmund Gettier's three-page 1963 paper showed that JTB can be satisfied while knowledge is absent, because the belief may be true for reasons unrelated to the believer's evidence. The field has been chasing a fix ever since.
Propositional knowledge — Knowing that something is the case, the kind of knowledge that can be put into a sentence. Philosophers contrast it with procedural know-how (riding a bicycle) and acquaintance knowledge (recognizing a friend's voice).
A priori knowledge — Truths we can grasp without checking the world — the fact that 7 + 5 = 12, or that no bachelor is married. Reflection alone does the work.
A posteriori knowledge — Claims that require observation: that the kettle is boiling, that copper conducts electricity, that the local bus was late this morning. Experience supplies the verdict.
Each of these terms marks a different pressure point on the same question: under what conditions does a thought in somebody's head actually count as knowing?
3. Grounds, Reasons, and Evidence
Justification is where epistemologists argue most. What has to be true of a belief, or of the person holding it, for the belief to count as well-supported?
Foundationalism — The picture that knowledge has a base layer of beliefs — perhaps sensory reports or self-evident truths — that need no further support, while everything else rests on inferences from that base.
Coherentism — The rival view that beliefs prop each other up like the beams of a geodesic dome: no single belief is foundational, but the whole web hangs together because its parts fit.
Reliabilism — A belief counts as justified, on this account, if it was produced by a process that usually yields truths — healthy eyesight, a calibrated instrument, sound reasoning — even if the believer can't articulate why.
Evidentialism — Justification tracks evidence. Your belief is as well-supported as the total evidence you currently have, no more and no less.
Infinite regress — The worry that every reason needs a reason behind it, which needs a reason behind it, and so on forever — a puzzle foundationalism, coherentism, and other theories each try to dissolve.
The stakes reach beyond seminar rooms. Courts, laboratories, and newsrooms all operate on some theory of what makes evidence strong enough to act on.
4. Where Knowledge Comes From
Knowledge reaches us through several channels. Epistemology asks how much each one can be trusted and what each one leaves out.
Perception — Information picked up through sight, sound, touch, taste, and smell. It is the most immediate route to facts about the surrounding world, and also the one that illusions and hallucinations complicate.
Reason — The inferential capacity that takes what we already accept and draws further conclusions, whether by strict deduction, by generalization from cases, or by inference to the best explanation.
Testimony — Knowledge that travels through other people's words: a friend telling you the train was canceled, a textbook reporting the year of a battle. Most of what any of us know comes this way.
Memory — The faculty that carries past learning forward. Without it, every morning would begin from scratch; with it, knowledge accumulates but risks quiet distortion.
Introspection — Turning attention inward to inspect one's own thoughts, moods, and sensations. Many philosophers have treated introspective reports as specially secure, though cognitive science has softened that confidence.
Intuition — The sense of immediately "seeing" that something is so, without a conscious argument. Whether intuitions deserve evidential weight is an open fight.
Each source has its blind spots, and much of epistemology consists of mapping exactly what those blind spots are.
5. Doubts and Skeptical Challenges
Skeptical arguments ask whether any of our usual claims to know hold up under pressure. Their job is less to convince than to make us say precisely what our knowledge rests on.
Skepticism — The stance that doubts whether genuine knowledge — or at least certain knowledge — is available at all, and that demands a defense from anyone who claims it.
Cartesian doubt — Descartes' method of deliberately rejecting every belief he could possibly doubt, in search of something he could not doubt and could rebuild from.
Radical skepticism — The harder line that nothing beyond one's own current mental states can be known, since every route to the outside world could in principle mislead.
Brain in a vat — A contemporary update of Descartes' demon: imagine your brain floating in nutrient fluid, wired to a computer that feeds it exactly the experiences you are having right now. What, if anything, rules that scenario out?
Pyrrhonian skepticism — An ancient tradition, associated with Sextus Empiricus, that responds to conflicting arguments by suspending judgment altogether, treating the tie between opposing views as evidence neither wins.
Whether or not one finds these challenges decisive, wrestling with them has forced every major theory of justification to get sharper.
6. What Truth Means
Before asking whether a belief is known, it helps to ask what would make it true. Philosophers have offered several accounts, each with its own payoff and price.
The main accounts
On the correspondence account, a statement is true when it matches how things actually stand — "snow is white" is true because snow is, in fact, white. The coherence account instead ties truth to fit: a claim counts as true when it dovetails with a larger, consistent body of accepted statements. The pragmatic account, developed by William James and John Dewey, treats truth as whatever proves durable and useful across inquiry and action. The deflationary account says that calling a sentence true does no more than reassert it: "'grass is green' is true" just means grass is green.
Terms that hang nearby
Objectivity — The ideal of judging by the facts rather than by personal preference, a standard that scientific and journalistic work aspire to even when they fall short.
Certainty — Total confidence in a belief, with no room left for doubt. Whether any empirical claim can really reach that level is itself a long-running debate.
Fallibilism — The acceptance that any belief, however well-grounded, might still turn out to be wrong, so that inquiry should never close the door on revision.
These terms let us describe what we are aiming at when we try to get things right, and why that aim is harder than it looks.
7. Rival Schools of Thought
Across centuries, thinkers have clustered into camps over where knowledge really comes from and how far reason alone can reach.
Empiricism — The thesis that experience is the original source of everything the mind contains, a line running through Locke, Berkeley, and Hume, who compared the newborn intellect to a blank page.
Rationalism — The opposing thesis that the most important truths are accessible to reason alone and that some structures of the mind are there from the start — the position defended by Descartes, Spinoza, and Leibniz.
Constructivism — The view that knowers actively build their knowledge rather than simply absorbing it, shaped by their mental equipment, their language, and their social setting.
Pragmatism — A distinctively American tradition that measures beliefs by their consequences in practice, treating the long-run success of an idea as the best indicator of its truth.
Kantian synthesis — Kant's attempt to settle the empiricist-rationalist quarrel: experience supplies the raw material of knowledge, but the mind's own forms — space, time, causality — organize that material into something intelligible.
These positions still frame current arguments, whether the topic is scientific realism, the status of mathematical objects, or the role of culture in cognition.
8. Knowing Together
Knowledge is rarely a solo achievement. Social epistemology studies the groups, institutions, and power relations that decide what gets accepted as known.
Social epistemology — The study of how communities — peer reviewers, juries, classrooms, online networks — produce, circulate, and vet knowledge together.
Epistemic injustice — Miranda Fricker's term for the harm done to someone as a knower: being disbelieved because of prejudice (testimonial injustice), or lacking the shared concepts needed to make sense of one's own experience (hermeneutical injustice).
Epistemic community — A group that shares methods, standards, and background assumptions for evaluating claims, such as a scientific specialty, a craft guild, or a court.
Epistemic authority — The standing that licenses a person or institution to be believed in their area — the reason you take a cardiologist's word about your heart more seriously than a stranger's.
Thinking socially about knowledge changes which questions look important: not just "is this true?" but "whose voice gets heard, and who decides?"
9. Classic Thought Experiments
Epistemologists often argue by way of vivid scenarios that test intuitions about knowledge and appearance. In Plato's Cave, prisoners chained in place see only shadows cast on a wall and take those shadows for the whole of reality — a parable about how impoverished experience distorts understanding. Descartes' Evil Demon imagines a cunning spirit devoted to fooling you about everything external, pressing the question of whether any belief about the world is secure against total deception. The veil of perception puts the same worry in a different key: if all we ever directly encounter are our own mental images, how do we reach past them to the things those images are supposed to represent? Russell's five-minute hypothesis asks whether you could rule out that the universe sprang into existence five minutes ago, memories and fossils included — a gentle reminder that evidence about the past has to be argued for, not assumed.
10. Current Debates
Modern epistemology has expanded to answer pressures that earlier philosophers could not have anticipated. Epistemic bubbles and echo chambers describe information environments — recommendation feeds, partisan media diets — that quietly filter out dissenting views and harden the opinions we already had. Misinformation and disinformation raise sharp questions about epistemic responsibility: what each of us owes before resharing a claim, and what platforms owe their users. Virtue epistemology shifts attention from abstract rules to the habits of a good thinker, prizing intellectual humility, curiosity, carefulness, and the courage to follow an argument where it goes. Formal epistemology imports probability, logic, and decision theory to model belief change and rational choice with mathematical precision. Naturalized epistemology, following W. V. O. Quine, urges that the theory of knowledge be woven into empirical science, treating human cognition as a phenomenon that psychology and neuroscience can help explain.
The vocabulary in this guide is the working kit of anyone who wants to think carefully about thinking. With information cheap and plentiful but trust in short supply, being able to name the distinction between belief and justification, between testimony and evidence, or between certainty and fallibilism is genuinely useful. Whether you are reading Plato for the first time, arguing with a search engine's answers, or just trying to figure out which source to trust on a medical question, this is the language that keeps the discussion honest.
Look Up Any Word Instantly on Dictionary Wiki
Get definitions, pronunciation, etymology, synonyms & examples for 1,200,000+ words.
Search the Dictionary